Composition

Composition is the creation of new classes, by including other classes, and delivering a specific interface over them.

<?php

class Wheel {
    int $diameter;
}

class GasTank {
    int $capacity;

    public function fillUp(int $amount) {
        $this->amount = $amount;
    }

    public function empty() {
        $this->amount = 0;
    }

}

// Car composes 2 other classes: GasTank and Wheel
// These object should be accessed via the Car class, not the direct objects
class Car {
    Wheel $wheels;
    GasTank $gasTank;

    public function fillUp(int $amount) {
        $this->gasTank->fillUp($amount);
    }

    // empty() the gasTank is now available via the car (just drive it, or and it to garage)
}

?>

Documentation

See also Composition Over Inheritance in PHP, Object composition in PHP with Example

Related : Inheritance